The invention relates to the exploitation of natural resources under the seabed, primarily the recovery of hydrocarbons (oil and gas) from formations under the seabed.
More specifically, the invention relates to a method for exploiting natural resources under the seabed, where a surface vessel is anchored almost geostationarily with the aid of a turret that is rotatable relative to the vessel about a vertical axis and has mooring cables extending to the seabed, and where drilling and/or production of a well in the seabed is carried out from the vessel.
The invention also relates to a system for drilling a well in a seabed, comprising a surface vessel, means for holding the vessel in a desired position above a point on the seabed, which means comprise a turret that is rotatable relative to the vessel about a vertical axis and which can be anchored to the seabed by means of a plurality of mooring cables, and a riser extending from the seabed to the vessel through which the well can be drilled using a drill string from a derrick in a drilling section on board the vessel.
Hydrocarbon recovery from floating structures (floaters) is well known today. Such floaters may be in the form of a ship or in the form of semi-submersible units. The first known floaters were barges which were anchored using a plurality of anchors in the seabed and from where drilling operations were carried out. As the move was made into deeper water, equipment was developed which made it possible to drill despite the motions of the vessel.
In areas of the world where climatic conditions are rather harsh it is in practice virtually impossible for a vessel to lie having a particular directional orientation because wind and current, at times straight from the side, produce forces which far exceed the capacity of the mooring, not to mention the problems which are due to the rolling of the vessel.
In view of this, attempts have been made to introduce turret-based mooring on drill ships so as to enable the ship to turn with the weather and wave direction. At the same time, drill ships were introduced that have so-called dynamic positioning (ships with no mooring which maintain their position with the aid of gauges, automatic controls and propellers) and where a turret was superfluous to need, and these have since been the dominant solution whenever traditional fixed anchoring was not acceptable.
However, turret-based mooring had its renaissance in the form of what is known as production ships (FPSO: floating production, storage and offtake). Like drill ships, these vessels are provided with a turret in order to obtain a weathercock effect. As the ships grew to a considerable size, 100,000 tonnes dead weight and more, it became desirable to place a drilling system on board. This was particularly the case at great ocean depths where separate drilling vessels gave rise to substantial costs, and was also due to the fact that a large FPSO can easily be designed to take aboard additional weight and provide additional space for a drilling system. The size of these ships means that in terms of motion they can be compared with today""s semi-submersible drilling rigs, especially if the drilling system were placed in the midship region.
Several methods and systems have been proposed for such arrangements. Common to all is that it is desirable to pass the drill string down from or through the geostationary turret. Obviously, the reason for this is that in doing so the same freedom is obtained to operate with full weathercock effect, even when drilling operations are underway.
Inevitably, a solution of this kind will be rather complex, as there will be a rather large accumulation of functions on a small, concentrated area by the turret: the actual turret with bearing and braking/turning mechanism, anchor winches, production risers with valves and transfer equipment, drill floor with derrick-well opening and safety valve handling, tension machines for the drilling risers and so forth.
In order to avoid this, a vessel has been proposed that has no turret but has pure dynamic positioning and flexible production risers which are suspended in the vessel in such manner that they do not come into conflict with the drilling area and drilling risers, which it is desirable to maintain geostationary relative to the seabed. The drawback of this design is that the ship can only achieve a weathercock effect of 80-90xc2x0 to either side of the nominal direction. This can be remedied to some extent by the ship having two bows and being able to turn 180xc2x0 in order to continue the weathercock motion, with the stem against the wind. In regions where the climate is harsh, there will, of course, be a huge fuel consumption in order to hold the ship in position, and also a not inconsiderable safety problem if difficulties in holding position arise. Solutions without a turret and based on a system with a plurality of taut mooring cables taken into the midship region have also been proposed. With the aid of fast, controlled winch operations, the vessel can to a certain extent follow the direction of the wind. It has been proposed that this variant should be equipped with a double bow (a bow at each end). When the vessel has turned on its moorings to an extreme point the need arises for a rapidly executed change in direction of 180xc2x0. Common to all double bow solutions is that there is a problem as regards the location of living quarters, escape routes, the location of flares and the natural ventilation of the drilling and process plant. A rapid 180xc2x0 turn-around operation in rough seas is not particularly attractive either.
One of the objects of the present invention is to provide an improved solution as regards the said unfavourable conditions.
The known turret solutions, with drilling and production through the turret, are not completely satisfactory, especially as regards space and safety considerations. A great improvement can be achieved by separating the drilling area on the vessel from the turret, which permits the use of a traditional swivel solution in order to bring the produced gas/liquid on board, and also makes it possible to use a drilling area shaped like a traditional drill ship (normally dynamically positioned).
Of course, a drilling area placed outside the turret gives rise to some new problem areas.
Drilling in deep water takes place through a riser extending from the wellhead on the seabed to the vessel. If drilling has to be stopped because of the weather conditions (normally because of excessive heaving), the first step will be to hang the drill pipe up inside the safety valve on the seabed. The next step will be to pull the drill string up or break it off in the safety valve, whilst the riser remains connected throughout. This situation can prevail until the next operation barrier, where there is a risk that the heaving of the vessel may exceed that which can be compensated in the so-called telescopic joint uppermost in the riser arrangement. Before this happens, the connection with the seabed must be broken. Traditionally, the riser is then broken on the top of the safety valve on the seabed in a suitable pipe connection, the so-called connector. The procedure that follows is that the riser is hauled up to run clear of the equipment on the seabed by means of the tension machines on the vessel. In this condition, with the full length of the riser hanging below the vessel, the vessel can then ride out the storm and re-tie the connection once the weather has improved.
In the stormy conditions described, a dynamically positioned vessel will have no difficulties as regards the weathercock turning of the ship since the riser hangs freely and will follow the movement of the vessel. A turret-moored vessel with a corresponding riser arrangement will only be capable of operating in such stormy conditions (with riser hanging below the vessel) if the drilling and riser arrangement is placed in and operated through an opening in the actual turret. The obvious reason is that the drilling arrangement in another position on the vessel will rapidly result in the riser being able to impinge on the mooring cables and possible production risers, both of which are geostationary, whilst the vessel and the drilling riser turn with the weather. A theoretical possibility is to pull up the entire length of the riser and thus run clear of the said obstructions. In the offshore industry this would be considered extremely inexpedient as such an operation can take up to four days at great ocean depths (1500-2000 m) and result in completely unacceptable efficiency (or lack thereof) and considerable risk.
Recently, there have been proposals for a riser which is capable of breaking relatively close to the vessel and is made having a buoyancy means and a blow out preventer valve at the point where the riser breaks. Use of such a riser facilitates a relatively rapid disconnection because the length of piping which must be pulled up is greatly reduced.
A particular object of the present invention, especially in areas (waters) where there are stable weather conditions, in particular sustained, dominant wind directions, is to facilitate drilling and production in deep water (e.g., 100 meters or more) in a risk-reduced manner using existing technology, with the drilling centre as an at least almost geostationary point.
According to the invention there is therefore proposed a method as mentioned above, characterised in
that the drilling of the well is carried out from a drilling section in the vessel at a distance from the axis of rotation of the turret;
that the drilling is carried out using a drill string through a riser extending from the seabed to the drilling section, which riser can be broken and shut off between the seabed and the vessel; and
that the vessel upon manipulation (hauling in and slackening) of the mooring cables is held in position so that the drilling centre is kept almost geostationary relative to the well during drilling operations.
According to the invention there is also proposed a system as mentioned above, characterised in that the rotating unit and the drilling section are arranged so as to be
spaced apart in the longitudinal direction of the vessel;
that the riser is arranged in a circular sector around the turret which is free of mooring cables and possible production risers;
that the turret consists of means for manipulating (hauling in/slackening) the mooring cables; and
that the riser is capable of breaking and being shut off between the seabed and the vessel.
Further features of the new method and system according to the invention are disclosed hereinafter.
Further features of the new method and system according to the invention are disclosed in the dependent method and apparatus claims respectively.